Lithium rechargeable cell with reference electrode for state of health monitoring

ABSTRACT

A battery management system includes one or more lithium ion cells in electrical connection, each said cell comprising: first and second working electrodes and one or more reference electrodes, each reference electrode electronically isolated from the working electrodes and having a separate tab or current collector exiting the cell and providing an additional terminal for electrical measurement; and a battery management system comprising a battery state-of-charge monitor, said monitor being operable for receiving information relating to the potential difference of the working electrodes and the potential of one or more of the working electrodes versus the reference electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit under 35U.S.C. §120 of U.S. application Ser. No. 12/210,812, filed Sep. 15,2008, which claims the benefit under 35 U.S.C. §119(e) of U.S.Application No. 60/993,802, filed Sep. 14, 2007; and U.S. PatentApplication No. 60/994,089, filed Sep. 17, 2007, the contents of whichare incorporated by reference herein in their entireties.

TECHNICAL FIELD

This application relates to monitoring state of charge and/or state ofhealth of batteries. More particularly, this application relates tobatteries, battery monitoring systems, and methods of improving batteryperformance by monitoring state of charge and/or state of health ofbatteries.

BACKGROUND

State of charge (SOC) monitoring is desirable or necessary in manybattery applications, including portable electronics products such aswireless communications devices and laptop computers, power tools,electric vehicles (including hybrid, plug-in hybrid, and all-electricvehicles), backup power systems, energy storage for power generationdevices such as solar or wind collectors or fuel cells or conventionalfuel-burning power sources, and the like. A battery, or string ofbatteries forming a battery pack, may be used over a limited range ofSOC or over a wide range including the entire capacity available fromthe battery.

Accurate knowledge of the state of charge (SOC) and state of health(SOH) of a battery is important for many applications, and especially sofor long-life, high charge or high discharge rate applications such ashybrid electric vehicles (HEVs), plug-in hybrid electric vehicles(PHEVs), and electric vehicles (EVs). In hybrid electric vehicles, it isespecially desirable to monitor the state of charge of the battery sinceoperation does not typically use the whole range of SOC and typicallyuses an SOC range that is centered around approximately 50% SOC, forexample, within approximately 10-90% or 40-60% of SOC. Monitoring SOCand SOH can be difficult if the voltage of the battery varies relativelylittle with the SOC, or if the voltage is time-dependent at a constantSOC, or if hysteresis of voltage occurs so that the cell voltage ischarge/discharge history-dependent.

There are a number of situations in which it is desirable to know thepotential of each electrode in an electrochemical cell with someaccuracy. The potential at any one electrode in a battery may undergoexcursions in normal operation that brings it close to a potential thatcan cause damage or degrade performance or life. For example, there maybe too high a potential at the positive electrode causing electrolytedegradation or an overcharged positive active material. In the case of alithium ion battery, the potential may be too low at the negativeelectrode causing lithium metal plating.

As another specific example wherein detailed knowledge of the electrodepotential is needed in a practical battery, consider a lithium ionbattery undergoing charging at high rates. Too high a charge rate ordegradation of the cell can cause the potential at the negativeelectrode to drop below that of lithium metal and cause lithium platingat the negative electrode, which degrades life and can create a safetyconcern. However, if the potential at the negative electrode were knownwith accuracy, a battery management system could be designed to ceasecharging of the cell before significant lithium plating occurs.

Another reason to monitor SOC accurately is to improve the life orsafety of the battery. Some battery chemistries become unsafe at toohigh a charge voltage, and many chemistries degrade faster at very highor very low SOC. An accurate SOC estimate is therefore useful foroptimizing the system for safety or long life.

Therefore, it may be critical to know with accuracy the potential ateach electrode. However, the cell voltage, while easily measured, givesthe difference in potential rather than the absolute potential, andincludes various polarization contributions which may differ inmagnitude between the positive and negative electrode, thereby makingdetermination of the electrode potential difficult. New performancedemands such as an HEV have created a need for better SOC/SOHmonitoring. Existing reference electrodes such as lithium metal may notbe suitable for lithium ion battery systems used under theabove-described demanding conditions due to insufficient stability andlife (e.g. drift of the reference potential) or unsuitable referencepotential.

SUMMARY

Materials, cell designs, and methods of using a reference electrodeincorporated into a battery are provided in order to provide improvedstate-of-charge (SOC) and state-of-health (SOH) monitoring over thelifetime of the battery. Simplified cell designs are provided having areference electrode without the need for an additional port in the cellcan or capping lids for the reference electrode terminal.

Reference electrodes have been used for electrochemical studies ingeneral, but have not been designed for the purposes of monitoringnegative electrode potential to reduce or prevent Li deposition uponhigh rate charging, or for lifetime cell monitoring. Batteries andbattery systems incorporating a reference electrode according to one ormore embodiments provide information useful in meeting the operatingrequirements that HEV, PHEV, and EV systems have in terms of power,charge/discharge rate, cycle life, and calendar life.

In one aspect, a battery is disclosed, comprising first and secondworking electrodes separated by at least one separator, where the firstworking electrode is in electrical connection with a first terminal, andthe second working electrode is in electrical connection with a secondterminal, one or more reference electrodes, and a can housing theworking electrodes and the one or more reference electrodes, wherein thecan is electrically isolated from the first and second terminals and iselectrically connected to the one or more reference electrodes toprovide terminals for the one or more reference electrodes.

In one or more embodiments, the battery is a lithium ion battery and theworking electrodes comprise electroactive materials capable lithiumuptake and release. In one or more embodiments, the battery comprises acylindrical cell of wound construction. In one or more embodiments, thebattery comprises a prismatic cell of wound or stacked construction.

In one or more embodiments of the lithium ion battery, the one or morereference electrodes are comprised of an electroactive material capableof multiphase existence to provide a substantially constant voltagebetween about 1 V and about 4 V with respect to Li/Li+. In otherembodiments of the lithium ion battery, the one or more referenceelectrodes are capable of interface with a battery management system forthe charging of the battery and for monitoring state of charge. In stillother embodiments of the lithium ion battery, the one or more referenceelectrodes have as little as about 0.001% and up to as much as about 20%of the coulombic capacity of the working electrodes. In yet otherembodiments, the one or more reference electrodes are selected from thegroup consisting of lithium titanium oxide, lithium transition-metalphosphate, lithium manganese spinel, with or without substituentelements, and alloys of lithium with metals such as tin, aluminum, andantimony. In further embodiments, the one or more reference electrodescomprise lithium titanium oxide. In some embodiments, the one or morereference electrodes comprise lithium iron phosphate. In otherembodiments, the battery is one of a plurality of batteries comprising abattery pack. In additional embodiments, the one or more referenceelectrodes are positioned at a location in the battery most susceptibleto lithium plating during charge. In yet other embodiments, the one ormore reference electrodes are located between the working electrodes. Inother embodiments, the one or more reference electrodes aresubstantially adjacent to the edge of the negative electrode, andprevented from contacting the negative electrode by a porous,electronically insulating layer. And in some embodiments, the activematerial for the one or more reference electrodes is coated onto atleast part of a wall of the can.

In some embodiments the can consists of a metal from the groupcomprising aluminum, copper, stainless steel, and titanium and the canprovides both the reference electrode and the reference electrodeterminal. In some embodiments exposed metal surfaces of the can arecoated with a nonporous electrically insulating coating. In additionalembodiments, the first and second terminals are located in upper andlower cover plates, respectively.

In some embodiments, the first and second terminals are electricallyisolated from the can via gaskets. In other embodiments, the one or morereference electrodes are wrapped in a porous electronically isolatingmaterial that is electrochemically inert. In some of these embodiments,the porous electronically insulating material is wetted by the batteryelectrolyte.

In some embodiments of the lithium ion battery the one or more referenceelectrodes are maintained within their two-phase stoichiometry over thecourse of repeated voltage measurements, by compensating for the currentpassed during voltage measurement.

Another aspect is a method of supplying power, the method comprisinginstalling the lithium ion battery disclosed above. In some embodiments,the method further comprises interfacing the one or more referenceelectrodes with a battery management system, charging the battery, andmonitoring the state of charge.

In some embodiments of the lithium-ion battery, the compensation occursby alternating measurement between the reference-to-negative electrodeand positive-to-reference electrode. In other embodiments, thecompensation occurs by periodically switching the connection of thevoltage leads between the one or more reference electrodes and one ormore working electrode. In yet other embodiments, the compensationoccurs by periodically passing current between the one or more referenceelectrodes and either the positive or negative electrode, with thedirection and amount of current determined by the amount of currentpassed during voltage measurement.

In other embodiments, the method of supplying power further comprisesmaintaining the one or more reference electrodes within their two-phasestoichiometry over the course of repeated voltage measurements, whereinthe maintaining occurs by compensating for the current passed duringvoltage measurement. In some embodiments of the method, the compensationoccurs by alternating measurement between the reference-to-negativeelectrode and positive-to-reference electrode. In other embodiments ofthe method, the compensation occurs by periodically switching theconnection of the voltage leads between the one or more referenceelectrodes and one or more working electrodes. In yet other embodimentsof the method, the compensation occurs by periodically passing currentbetween the one or more reference electrodes and either the positive ornegative electrode, with the direction and amount of current determinedby the amount of current passed during voltage measurement.

In some embodiments of the lithium ion battery, the positive and/ornegative electrodes are comprised of materials which posses intrinsichysteresis greater than 1 mV.

Another aspect discloses a lithium ion battery system, comprising (a)one or more lithium ion cells in electrical connection, each said cellcomprising first and second working electrodes separated by separatormembranes, the working electrodes capable of lithium ion uptake andrelease, the first working electrode comprising a first electroactivelayer on a first current collector, and the second working electrodecomprising a second electroactive layer on a second current collector,and one or more reference electrodes, each reference electrodeelectronically isolated from the working electrodes and having aseparate tab or current collector exiting the cell and providing anadditional terminal for electrical measurement, and (b) a batterymanagement system comprising a battery state-of-charge monitor, saidmonitor being operable for receiving information relating to thepotential difference of the working electrodes and the potential of oneor more of the working electrodes vs. the one or more referenceelectrodes.

In some embodiments of the system, the one or more reference electrodesare comprised of an electroactive material capable of multiphaseexistence to provide a substantially constant voltage between about 1 Vand about 4 V with respect to Li/Li+. In other embodiments of thesystem, the one or more reference electrodes are selected from the groupconsisting of lithium titanium oxide, lithium transition metalphosphate, lithium manganese spinel, and alloys of lithium with metalssuch as tin, aluminum, and antimony. In yet other embodiments, the oneor more reference electrodes are positioned at the position of the cellmost susceptible to lithium plating during charge. In additionalembodiments, the one or more reference electrodes are located betweenthe working electrodes. In other embodiments, the one or more referenceelectrodes are immediately adjacent the edge of the negative electrode,and are prevented from contacting the negative electrode by a porous,electronically insulating layer. In yet other embodiments, the one ormore reference electrodes are encapsulated by porous polyolefinseparators.

In other embodiments of the system, the one or more reference electrodesare coated by a porous insulating coating comprised of a mixture ofceramic particles and binder, said ceramic particles consisting of SiO2,Al2O3, MgO, TiO2, or other electrically insulating ceramic material, andsaid binder consisting of poly(vinylidene difluoride),poly(tetrafluoroethylene), poly(ethylene), poly(ethylene oxide),poly(methymethacrylate), latex rubber, carboxy methyl cellulose, orother polymer. In some embodiments, all metal connected to the one ormore reference electrodes, except the metal immediately covered by theactive reference electrode material, is insulated with a nonporouselectrically insulating coating. In other embodiments, the porousinsulating layer has a thickness between 5 and 100 micrometers.

In one or more embodiments of the system, the one or more referenceelectrodes are placed in close proximity to the working electrodes, suchthat the surface of the insulating layers around the one or morereference electrodes contacts the separator separating the positive andnegative electrodes. In additional embodiments, the battery is acylindrical, prismatic or pouch battery. Other embodiments includesensors for monitoring temperature and/or current. In some embodiments,the state-of-charge monitor can monitor one or more parameters selectedfrom the group comprising overcharge, overdischarge, excessive chargecurrent, and excessive discharge current.

One or more embodiments also contain a balancing module. In some ofthese embodiments, the one or more lithium ion cells comprise cellpairs, and wherein the balancing module can evaluate the relativevoltage levels of adjacent cell pairs and redistribute charge betweenadjacent cells to mitigate differences in the cell voltages of thepairs.

Some embodiments of the system also include a controller. In some ofthese embodiments, the controller can raise and/or lower the charge rateof one or more cells.

In one or more embodiments of the system, the one or more referenceelectrodes can allow substantially instantaneous feedback of thestate-of-charge of each individual cell to the battery managementsystem. In some of these, the battery management system can adjust thecharging protocol of at least one cell in substantially real-time.

In some embodiments of the system, the system can estimate thestate-of-charge. In some of these embodiments, the system can comparethe estimated state-of-charge to a target state-of-charge, and whereinthe battery management system can adjust the charge rate of at least oneof the cells upwards or downwards.

Another aspect is a method of avoiding lithium plating in a lithium ionbattery comprising measuring the potential of the negative electroderelative to a reference electrode during charging of a lithium ionbattery, comparing the measured potential to a critical potentialassociated with the plating of lithium metal, and adjusting the chargingconditions of the lithium ion battery to reduce the risk or preventplating of lithium at the negative electrode. Some embodiments of thismethod include adjusting charging via terminating charging or alteringthe charge rate.

Another aspect is a method of minimizing charge time of a lithium-ionbattery by maximizing the charge current that is applied at anyparticular SOC during a charging event, comprising measuring thepotential of the negative electrode relative to a reference electrodeduring charging of the battery, said charging having a charge rate,determining the state of charge of the battery, comparing the measuredstate of charge to a state of charge profile, and adjusting the chargerate upwards or downwards to maintain the actual charge rate within apredetermined range that provides one or more of optimal safetyoperation and optimal charge rate to minimize charge time.

Another aspect is a method of detecting whether there is an electricalconnection between a can and either terminal of a cell, comprisingapplying a material to the inside of the can, said material having aredox potential that differs from that of either terminals, where thepotential difference is greater than 0.2 V, and measuring the voltagebetween at least one terminal and the can.

Another aspect is a method of supplying power, the method comprisingimplementing a lithium ion battery system selected from the groupconsisting of earlier disclosed lithium ion battery systems. In some ofthese methods, the lithium ion battery system also monitors one or moreparameters selected from the group comprising overcharge, overdischarge,excessive charge current, and excessive discharge current. In one ormore embodiments, the method includes evaluating the relative voltagelevels of adjacent cell pairs, and redistributing charge betweenadjacent cells to mitigate differences in the cell voltages of thepairs. In other embodiments, the method includes raising and/or loweringthe charge rate of one or more cells. In yet other embodiments, themethod includes adjusting the charging protocol of at least one cell insubstantially real-time. In yet other embodiments, the method includesestimating the state-of-charge.

FIGURES

A more complete appreciation of the invention and many of its advantageswill be understood by reference to the description of the invention whenconsidered in connection with the following drawings, which arepresented for the purpose of illustration only and are not intended tobe limiting. Other embodiments and modifications within the reach of oneof ordinary skill in the art are intended to be included in theinvention.

FIG. 1 is an exemplary electrochemical cell including a referenceelectrode according to one or more embodiments.

FIG. 2 is an exemplary battery pack according to one or moreembodiments.

FIG. 3 is a flow diagram illustrating a method for monitoring a batteryor battery pack according to one or more embodiments.

FIG. 4 is a flow diagram illustrating a method for monitoring a batteryor battery pack according to one or more embodiments.

FIG. 5 is an exemplary electrochemical cell including a referenceelectrode according to one or more embodiments.

FIG. 6 shows a voltage profile of an exemplary electrochemical cellincluding a reference electrode during charge and discharge; the graphshows the potential of the negative electrode measured against thereference electrode, and also the cell voltage (positive vs. negativeelectrode). The reference electrode is used to determine when toterminate charging, in order to prevent lithium plating.

FIG. 7 is a perspective illustration of a typical spiral electrodesecondary cell according to one or more embodiment.

FIG. 8 is a perspective illustration of a cylindrical cell including areference electrode according to one or more embodiment.

FIG. 9 is a perspective illustration of a cylindrical cell fabricatedwith a reference electrode terminal through the cell endcap according toone or more embodiment.

FIG. 10 illustrates the voltage profile during charge from a cylindricalcell in which the terminals are electrically isolated from an aluminumcan, and the can is used as a pseudoreference electrode.

FIG. 11 shows the voltage profile from a cell containing an LTOreference electrode during an HPPC test. By comparing the voltage dropin the cell voltage to that in the neg. v. ref. voltage upon change incurrent, the impedance of the entire cell can be compared versus that ofthe negative electrode.

FIG. 12 shows the voltage profile of a cell containing a lithiumreference electrode, starting fully charged, discharging in 5% SOCincrements to 0% SOC, then charging in 5% SOC increments to 100% SOC,with 2 hour rests between each current event.

FIG. 13A-C provide an exploded view of the components used in a negative(anode) end cap assembly, including gasket for insulating negative andreference electrode terminals.

FIGS. 14A and 14B are perspective illustrations of a cylindrical cellincluding a reference electrode according to one or more embodiments.

DETAILED DESCRIPTION

Three-electrode cells provide a means to monitor the state of anelectrochemical cell to obtain information about cell characteristics.This information can be used to determine the state of charge of abattery and other important cell characteristics. Such information isincreasingly needed to monitor and optimize complex battery systemsinvolving multiple cells with tight operating parameters, such as arefound in PHEVs.

While three-electrode cells are useful in monitoring and optimizing cellperformance, such systems pose certain challenges. For example, the useof an additional electrode increases the complexity of cell design. Inparticular, it requires reengineering of the cell architecture toaccommodate a third electrode and its terminal, typically requiring anadditional port in the cell container. See, e.g., EP 1577914. Theadditional port unnecessarily complicates the cell design and providesadditional safety risks, as the port is an additional site for ruptureand leakage.

In one aspect, a three electrode battery and battery system is providedwith improved performance and a simplified design that does not requireadditional modification of the cell can design to accommodate the thirdelectrode.

Methods for the operation, monitoring and optimization of the cellperformance are also provided.

Electrochemical Cell and Battery

An electrochemical cell including one or more reference electrodes isdescribed with reference to FIG. 1. The electrochemical cell can be ofany geometry, such as a cylindrical cell of wound construction, aprismatic cell of wound or stacked construction, and the like. Theelectrochemical cell may be small or large, ranging in volume from lessthan 1 cm³ to greater than 1 liter, and have charge capacity rangingfrom less than 0.1 Ah to greater than 100 Ah.

One or more electrochemical cells can be formed into one or morebatteries. The battery can be of any geometry. For example, the batterycan be a prismatic battery, a cylindrical battery, or the like. Forexample, lithium ion batteries are typically included in a battery pack,which includes a plurality of electrochemical cells that areelectrically connected in series and/or in parallel. Lithium-ion batterypacks come in all shapes, sizes, energy capacity and power rating,depending on the intended use of the battery pack. Battery packs willtypically include a number of lithium ion cells and a battery managementsystem.

All the various types of electrochemical cells and batteries/batterypacks thereof are within the scope of the invention. However, it isdescribed herein for simplicity with reference to a simple pouchelectrochemical cell.

As shown in FIG. 1, the electrochemical cell can include a negativeelectrode 122 and a positive electrode 124 electronically separated by aseparator 126. The negative and positive electrodes (122, 124) haveseparate tabs (130, 132) that serve as terminals for electrical contactwith an external circuit. The cell also includes a reference electrode134 that is electronically isolated from the working electrodes (122,124) and has a separate tab 136 exiting the cell and providing anadditional terminal for electrical measurement and control of thereference electrode. In certain embodiments as described more fullyherein, the terminal for the reference electrode is the conducting canhousing the cell, thereby avoiding the need to have an additional portin the battery. In certain embodiments, the reference electrode may bewrapped in a porous electronically insulating material (not shown) thatis electrochemically inert and is wetted by the electrolyte, e.g.microporous polyethylene, a mixture of insulating ceramic particles withbinder such as TiO₂ or Al₂O₃ with PVDF or other polymer binder, or othermaterial commonly used for battery separators.

Positive Electrodes

The working electrodes can be any conventional positive and negativeelectrodes. By way of example, suitable materials for the positiveelectrode for a lithium ion battery include LiCoO₂, LiNiO₂, LiMnO₂,LiMn₂O₄, LiFePO₄, V₂O₅, or other such cathodes well-known to thoseskilled in the art, including Li_(x)MO₂, in which M can include Ni, Co,Mn, Al, Mg, Cr, or other metals, mixed with a binder and optionally aconductive additive such as carbon.

Lithium metal phosphate compounds (such as lithium-transitionmetal-phosphate compounds) may also be used as the electroactivematerial including but not limited to polyanion compounds such asolivine compounds and NASICON compounds. The lithium-metal-phosphatecompound may be optionally doped with a metal, metalloid, or halogen.

Specific examples can include doped nanophosphate material, or anolivine structure compound LiMPO₄, where M is one or more of V, Cr, Mn,Fe, Co, and Ni, in which the compound is optionally doped at the Li, Mor O-sites. Deficiencies at the Li-site can be compensated by theaddition of a metal or metalloid, and deficiencies at the O-site can becompensated by the addition of a halogen.

Further information regarding suitable positive electrode materials canbe found in U.S. Pat. No. 7,338,734, entitled “CONDUCTIVE LITHIUMSTORAGE ELECTRODE” and United States Published Application No.2007/0031732, entitled “NANOSCALE ION STORAGE MATERIALS” which areincorporated by reference in their entireties.

Suitable positive electrode materials for batteries with aqueouselectrolytes, such as Pb-acid or nickel-cadmium batteries, include leaddioxide, nickel oxyhydroxide, and manganese dioxide.

Negative Electrodes

Suitable materials for a negative electrode for a lithium ion batteryinclude carbon, including graphitic or amorphous or partially disorderedcarbons, alloys or compounds formed between Li and metal alloyscomprising one or more of Sn, Si, Sb, Al, Zn, and Ag, or other anodematerials known to those skilled in the art.

Suitable negative electrodes for batteries with aqueous electrolytes,such as Pb-acid or nickel-cadmium batteries, include lead, cadmiumhydroxide, metal-hydride alloys, zinc, and carbon.

Reference Electrode

The selection of materials for a reference electrode will vary in thevarious secondary batteries such as lead-acid or Pb-A battery,alkaline-manganese battery, nickel-cadmium or ‘NiCad’ battery,nickel-metal hydride or ‘NiMH’ battery and the lithium-ion or ‘Li-ion’battery.

In certain embodiments, the reference electrode can have a coulombiccapacity that is small in comparison to the working electrodes, e.g.,the anode and the cathode which supply the coulombic capacity of theelectrochemical cell. Coulombic capacity is the amount of coulombs(current times time) that can be exchanged between the electrodes. Inone or more embodiments, the reference electrode has as little as about0.001% and up to as much as about 20% of the capacity of the workingelectrodes. In some embodiments, the reference electrode can occupy asmall section of that total volume in order to avoid substantiallydecreasing the volumetric energy density of the cell.

The reference electrode material may be one of many choices. In certainembodiments, the reference electrode can include a material providing astable reference potential over time in the cell environment. Thereference electrode can be thermodynamically stable in theelectrochemical environment of the cell.

For example, for lithium-ion batteries containing carbonate, ester,ether, lactone, or similar solvents, a stable reference potential can beat a high enough absolute potential that surface reactions due toelectrolyte reduction (such as the well-known “solid-electrolyteinterface” (SEI)) do not occur. This can be accomplished by having areference electrode with a potential greater than about 0.8V withrespect to lithium metal (Li/Li⁺), more preferably greater than about1.0V. Furthermore, the potential can be lower than about 4V vs. Li inorder to avoid oxidative reactions with components of the electrolyte.In batteries containing an aqueous electrolyte, the reference electrodecan be chosen to a potential greater than about 0 V vs. H₂/H⁺ and lessthan about 1.2 V vs. H₂/H⁺, i.e., within the stability window of water(which is a function of pH).

In another example, the reference electrode can include a material inwhich the potential is relatively constant even if the referenceelectrode is partially lithiated or delithiated while in use, such as areference electrode in which the chemical potential is constant withdegree of lithiation. Materials having a substantially constant chemicalpotential with degree of lithiation can include materials having morethan one co-existing lithium-active phase, such as compounds thatundergo a two-phase reaction upon insertion or removal of lithium. Suchcompounds can have a constant thermodynamically-determined potential asdetermined by the Gibbs phase rule. Such materials can be available witha variety of potentials, and can make desirable reference electrodes.

Exemplary (and non-limiting) reference electrode materials for Li ionbatteries can include lithium titanium oxides (LTO), lithium transitionmetal phosphates, lithium manganese spinels (on the ˜3V voltage plateauin between compositions LiMn₂O₄ and LiMnO₂), and alloys of lithium withmetals such as tin, aluminum, and antimony. In other embodiments lithiummetal may be used. Rutile structure compounds including Li_(x)RuO₂ andLi_(x)TiO₂, and alkali transition metal polyanion compounds, includingpure or doped compositions of the compounds Li_(x)MPO₄, Li_(x)MP₂O₇,Li_(x)MPO₄F, Li_(x)M₂(SO₄)₃ and Li_(x)M₂(PO₄)₃ in which M is one or moreof Ti, V, Cr, Fe, Mn, Ni or Co, and in which other alkali metals may bepartially substituted for Li, all are suitable electroactive materialsfor the reference electrode. The reference electrode active material maybe the same as, or different from, that used in one of the workingelectrodes of the battery. For lithium rechargeable batteries, lithiummetal is one usable reference electrode material.

In one or more embodiments, a multiphase lithium active materialproviding a stable constant potential can be used as the referenceelectrode. Lithium titanium oxide (LTO) is an exemplary referenceelectrode material including, but not limited to, a compositionLi_(4+x)Ti₅O₁₂ and compounds having the spinel structure. Upon lithiuminsertion, this composition can undergo a two-phase reaction and canprovide a constant 1.55V potential at room temperature vs. Li/Li⁺, whichis high enough to avoid SEI formation. This compound can be prepared inthe oxidized state. Alternatively, a two-phase material may be preparedby increasing the Li content and firing in reducing ambient atmosphereor in a closed-system firing where the relative amounts of Li, Ti and Oare constrained. When using the oxidized form, upon incorporating thereference electrode into a lithium ion cell, LTO may not be a two-phasematerial of constant potential. However, in certain embodiments, thereference electrodes can be electrochemically lithiated by the insertionof lithium to form the two-phase state of constant potential prior touse as a reference electrode.

Other exemplary reference electrode materials can include phosphatematerials, such as doped nanophosphate material, or olivine LiMPO₄ inwhich M includes one or more of Fe, Mn, Co, and Ni. The aforementionedphosphates can be lithiated to form a two-phase material of constantpotential. In such embodiments, the reference electrode can be operatedto avoid large cycling of the reference electrode, in order to keep thestoichiometry of the reference electrode within its two-phase region,and in order to avoid inducing changes in the potential of the referenceelectrode owing to the hysteresis intrinsic in some phase-changematerials. The two-phase phosphate can be prepared as a startingmaterial, or a starting reference electrode can be electrochemicallylithiated or delithiated to form the two-phase material. As anon-limiting example, a two-phase LiFePO₄—FePO₄ mixture with a potentialof about 3.45 V vs. Li/Li+ can be prepared by starting with an overallcomposition Li_(1−x)FePO₄ in which x is greater than about 0.05, andheat treating to create the two co-existing phases. Alternatively, thereference electrode can be LiFePO₄ that is delithiated in-situ afterassembly to create the two-phase mixture. In yet another example, thereference electrode can be FePO₄ that is lithiated in-situ afterassembly to a composition Li_(y)FePO₄ in which y is greater than about0.05.

Because voltage measurements involve passage of some small amount ofcurrent, over extending voltage measurements, the stoichiometry of thereference electrode can be changed, even to the point at which all ofthe capacity of the reference electrode is consumed, after which thepotential of the reference electrode will change. For a system designfor long-term operation, one would operate the voltage measurement toavoid depleting the reference electrode. Examples of modes of operationthat avoid substantially changing the stoichiometry of the referenceelectrode include reducing the amount of time during which voltage ismeasured, alternating the direction of the current passed during voltagemeasurement (i.e., by switching the polarity of the leads), and/orperiodically passing compensating current between the referenceelectrode and one of the working electrodes.

The active material of the reference electrode can be deposited directlyonto a metal current collector. Or, particles of active material can bemixed with a binder, conductive additive such as carbon may be added tothe mixture, and coated onto metal foil. The metal foil may be copper,aluminum, nickel, stainless steel, titanium, or other metal that neitheralloys nor corrodes under the operating potential window of thereference electrode.

Because the function of the reference electrode is to provide a stablemeasurement of the potential in a particular location in the cell, it isimportant to minimize factors which can affect that potential. Forexample, the metal lead connecting the active reference material to thereference terminal should be insulated from ionic contact with theelectrolyte in all regions except the immediate region of the activereference material. In addition, the reference electrode should beplaced as close as possible to the location where the potential is ofinterest, and there should be a continuous ionic path between thelocation of interest and the reference electrode. For example, if thegoal is to detect lithium plating, then the reference electrode shouldbe placed as close as possible to the negative-electrode tab, and morespecifically, to the separator-negative electrode active-materialinterface adjacent to the negative-electrode tab.

In one or more embodiments, the reference electrode has its own terminalwith which to connect a voltmeter. In one or more embodiments, thereference electrode lead connecting the reference electrode activematerial to the reference electrode terminal passes through afeedthrough or port through the cell wall or one of the upper or lowercaps on the cell can. The feedthrough through the cell must behermetically sealed. Such a seal can be made hermetic via a gasket,glass-to-metal seal, lamination, or other techniques known in the art.

In one or more embodiments, the reference terminal is integral with theone of the cell wall or upper or lower caps, so that an additional portor feed through is not required. For batteries with one or morereference electrode terminals, integration of the reference electrodeterminals into a cell wall or an end cap provides several significantadvantages over batteries that accommodate reference terminals via afeedthrough or port. Cell design of a feedthrough for a referenceterminal can be complicated, costly, and less durable. The hermetic sealnecessary for a feedthrough for a reference terminal adds a potentialleak path to the system. Failure in such a hermetic seal could result ina battery that leaks electrolyte and could cause the entire battery tofail. The structure required for such a reference terminal feedthroughalso takes up additional space in the battery. Further, such a referenceterminal feedthrough adds extra weight to the battery.

Integration of a reference electrode terminal into a cell wall or anendcap makes an additional feedthrough unnecessary. As a result, a newpotential leak path is not created, the space necessary to accommodatethe additional feedthrough is saved, and the weight of an additionalfeedthrough is also saved. One result of a battery with a referenceelectrode terminal integrated into a cell wall or an endcap is thus thatthe battery can be lighter, smaller, and more durable because of thelack of an additional potential leak path.

Another advantage of embodiments that have a reference terminalintegrated into a cell wall or an endcap is additional flexibility inthe structure required to measure voltages. Because the referenceterminal of these embodiments is electronically connected to the can,and because the can and the endcaps are electronically conductive,practically any point on the can or the endcaps can be used to measurethe potential of the reference electrode. This feature providessignificant flexibility in cell design, as potential across thereference electrode can be measured practically anywhere on a surface ofthe can or endcap.

Certain embodiments have one or more reference electrodes integratedinto the can. In embodiments in which the reference electrodes areelectronically connected to the can, the entire can effectively becomesthe reference terminal. When the can effectively becomes the referenceterminal, the can must be electrically separated from the endcaps inorder to maintain the electrodes at different potentials. Gaskets thusprovide electrical separation between the cell endcaps and can 830.Further information regarding suitable gaskets can be found inco-pending U.S. application Ser. No. 11/515,597, entitled “BATTERY CELLDESIGN AND METHOD OF ITS CONSTRUCTION” which is incorporated byreference in its entirety.

FIG. 13 is an illustration of an endcap including a gasket that may beused according to one or more embodiments of the invention toelectronically isolate the positive and/or negative terminal from thecan. FIGS. 13A-13C depict a negative end cap (5) including a centrallylocated fill hole (40). The fill hole is used to activate the cell onceassembled and is defined, at least in part, by a hollow bore rivet (45)which makes up the power terminal. Dual use of the central location ofthe negative end cap as both a fill hole and as a power terminalprovides efficient use of space and does not interfere with batteryoperation. The fill hole (40) is centrally located in the end cap face.The centrally located fill hole provides a feed through inlet fittinglydisposed within the hole and connecting to the interior of the cell.Electrolyte is introduced through this feed through inlet duringactivation.

The negative end cap is constructed by assembling the constituentcomponents as illustrated in the exploded diagram of FIG. 13A. Uppergasket (44) is placed into end cap body (43), which may contain adepression for receiving the gasket. The hollow bore rivet serving asthe power terminal (45) is assembled into upper gasket (44). The stem(45 a) of rivet (45) extends through a central opening of both the uppergasket (44) and end cap body (43). The assembly is flipped over, andseal gasket (47) is inserted onto gasket (44) and placed onto body (43).Lower gasket (42), seal gasket (47), and rivet backing disc (46) areassembled and positioned as illustrated in FIG. 4A. Extension tab (41)is inserted onto the stem of rivet (45). The as-assembled components,prior to crimping are shown in FIG. 13B.

Rivet (45) may be Ni plated steel for both good corrosion resistance andgood weldability, which serves as the power terminal for the cell. Theflat head of rivet (45) extends over a portion of the external face ofthe end cap and the hollow stem (45 a) extends into the interior of thecell. It also includes a fill hole through its center with an engineeredledge to help sealing, a symmetric shape, and a centralized rivet stemfor sharing space and symmetry between the battery terminal and the fillhole. Extension tab (41) connects the power terminal (45) with thecell's internal active anode material. A lower gasket (42) protects theextension tab (41) from contacting the end cap body (43), which is at adifferent voltage potential. Body (43) is hermetically sealed to thebattery tube (not shown) or the main body of the cell through any numberof methods, including but not limited to the aforementioned methods ofcrimping and welding. Upper gasket (44) insulates the power terminal(45) from the end cap body (43), which are at different voltagepotentials. Rivet backing disc (46) helps to create a robust press-rivetclamp force onto body (43). Seal gasket (47) helps to achieve a robustseal underneath the press-rivet.

The entire assembly may be crimped together by pressing and deformingthe stem of rivet (45), as illustrated in FIG. 13C, squeezing all of theparts together to form press-rivet (48) and creating a good electricalcontact between the extension tab (41) and the power terminal (45).

The same technique can be applied to create the positive terminal of thecell. However, at the positive terminal of the cell, rivet (45),extension tab (41), and rivet backing disc (46) are preferably comprisedof aluminum, aluminum alloy or a material resistant to corrosion at thepositive cell potential. Such materials may include stainless steel,molybdenum, hastelloy, or other known corrosion resistant alloys.

Other methods and gaskets may be employed as will be readily apparent toone of skill in the art.

The addition of gasketed endcaps in order to electrically isolate theworking terminals from the can in some embodiments that integrate thereference terminal into the can does not eviscerate the savingsattributable to the integrated reference terminal. The weight, spacerequirements, and leakage potential of gasketed endcaps are all smallerthan for an additional feedthrough to accommodate a reference electrodeterminal, thus retaining the benefits of the integrated referenceterminal in these embodiments.

Another concern when a reference electrode is added to a battery is thatthe reference electrode must be insulated from the working cells. Thistypically requires an insulating material to be placed around thereference electrode or between the reference electrode and the workingelectrodes to isolate the components in the battery. In embodiments inwhich the reference terminal is integrated into the can or the endcapsby having the active material for the reference electrode coated onto atleast a part of a can wall or endcap, the process of insulating theactive material of the reference electrode is simplified, as the coatedactive material need only be covered by the insulation.

Another benefit in embodiments in which a reference electrode isintegrated into the can or an endcap is maintaining contact between thereference electrode and the battery electrolyte. This is because theelectrolyte is in fluid contact with the interior of the can andendcaps, and thus will also be in contact with the reference electrodein certain embodiments.

In still other embodiments, the can itself may serve as apseudoreference electrode, further simplifying the design of the cell.In instances where the can is a metal such as aluminum, copper,stainless steel or titanium, the can is capable of serving as thereference electrode. The interior wall surface of the can be coated withprotective insulating material to provide electronic insulation of thecan from the working electrodes. Exemplary protective insulatingmaterials that may be used to coat the inside surface of the can includepolymers such as polyisobutylene, polyolefin, and epoxy, or ceramicssuch as alumina and zirconia. A suitable connection can be made at anypoint along the exterior wall of the can to create a circuit capable ofmeasuring the potential of the reference electrode v. negativeelectrode.

Advantages of the Reference Electrode

The incorporation of a specific reference electrode provides a range ofmonitoring capabilities to the electrochemical cell. The informationobtained by the reference electrode can be provided to a batterymonitoring system. Accurate measurement of the electrode potentialpermits accurate determination of the cell's state of charge, sincestate of charge is directly correlated to the potential differencebetween either working electrode and the reference electrode.Furthermore, for systems in which the impedance is SOC-dependent,accurate measurement of the electrode potential also providesinformation about electrode impedance. The battery monitoring system canuse this and other information, such as cell voltage, current, andtemperature, to control the various functions of the individualelectrochemical cells and the overall battery system. Battery managementsystems can therefore be developed to accurately monitor the state ofcharge and implement management of the battery state of health based onthis information. In one or more embodiments, battery monitoring systemsare provided that monitor the negative electrode potential to prevent Lideposition upon high rate charging. In other embodiments, batterymonitoring systems are provided to minimize charge time by maximizingthe charge current that is applied at any particular SOC.

1. State of Charge Determination

The state of charge (SOC) is defined as a percentage of the capacitythat the battery exhibits between a lower voltage limit at which thebattery is fully discharged at equilibrium, and an upper voltage limitat which the battery is fully charged at equilibrium. Thus a 0% SOCcorresponds to the fully discharged state and 100% SOC corresponds tothe fully charged state. State of health (SOH) is a measure of thebattery's present ability to deliver power and energy, and typicallyincludes information related to changes in cell impedance and capacity.

A voltage can be imposed across the positive and negative electrodes bycreating a circuit 150, or electrical connection, between the twoterminals 130, 132. The voltage is the difference between the potentialsof the positive and negative electrodes. Although the difference can beobtained, the absolute values of a single electrode cannot be measured.Generally, the cell voltage is suitable in determining SOC for theelectrochemical cell if one or both of the electrodes shows a change inpotential with SOC. In fact, SOC is conventionally determined in thismanner. However, there are circumstances in which the cell voltage is apoor indicator of cell state of charge. For example, in the case whereboth electrodes show a variation of potential with state of charge, butthe cell undergoes degradation reactions that change the stoichiometryof either or both electrodes over time, then cell voltage is no longer areliable indicator of the state of charge of the electrodes. Intrinsichysteresis in either or both working electrodes can also introduceuncertainty in the relationship between cell voltage and state ofcharge. Finally, polarization induced by the passage of current changesthe cell voltage; a reference electrode enables one to identify how muchpolarization is attributed to one or the other electrode.

Intrinsic hysteresis in either or both working electrodes can causeproblems using voltage to monitor SOC. This is because in materials withhigh hysteresis, SOC at a given voltage can be a function of the chargeand/or discharge history. The use of a reference electrode can allow asystem with intrinsic hysteresis to monitor SOC more accurately byseparately monitoring the potential of the positive and/or negativeelectrodes against the reference electrode. This is because there isless hysteresis in the potential of the negative electrode vs. referenceelectrode or positive electrode vs. reference electrode than in theoverall cell voltage, which contains contributions to hysteresis fromboth the negative and positive electrode.

Accordingly, in certain embodiments, the potential of the positiveand/or negative electrodes can be determined or controlled using areference electrode. In certain embodiments, the reference electrode canbe chosen so that the variation in potential as a function of itsstoichiometry at the reference electrode is much more stable than thevariation of potential at the negative or positive electrodes. In someother embodiments, the variation in potential as a function of SOC atone or both of the working electrodes is much more stable than thevariation of potential at the reference electrode.

For example, a circuit 152 may be created between the referenceelectrode 134 and the negative electrode 122, where the referenceelectrode is much more stable to variation in potential as a functionits stoichiometry than the negative electrode. Because the referenceelectrode is selected for its stability, the measurements and changes inpotential can be indicative of conditions at the negative electrode andSOC can be determined by measuring the difference.

As another non-limiting example, a circuit may be created between thereference electrode 134 and the positive electrode 124, where thereference electrode is much more stable to variation in potential as afunction of SOC than the positive electrode. Because the referenceelectrode is selected for its stability, the measurements and changes inpotential can be indicative of conditions at the positive electrode andSOC can be determined by measuring the difference.

In certain embodiments, the reference electrode can be located withinthe cell at a location where the potential varies the greatest, or wherethe consequences of undesirable potential excursions are the mostsevere. For example, in a large cell, where the electrical potential andpolarization and temperature may vary from location to location, thereference electrode may be situated at locations where such variationsare the most extreme. For example, the reference electrode can be placedjust outside the active area of the negative electrode, where lithiumplating upon rapid charging can occur.

In some other embodiments, one or more reference electrodes can bedistributed at various locations within a single cell to monitor thespatial variations in the electrochemical potential in the electrolyte.Each reference electrode would have an independent terminal.

2. Battery Management System

As discussed above, battery packs can include a number ofelectrochemical cells and a battery management system. The batterymanagement system can include sensors for monitoring temperature,current, and voltage, a voltage converter and regulator circuit tomaintain safe levels of voltage and current; an electrical connectorthat lets power and information flow in and out of the battery pack, anda battery charge state monitor, which estimates the present state ofcharge of the battery. The battery monitors can further accumulate datarelated to battery parameters and then transmit it to a host processor.Battery monitors can include mixed signal integrated circuits thatincorporate both analog and digital circuits, such as one or more typesof digital memory and special registers to hold battery data.

Some exemplary parameters that can be monitored by the batterymanagement system include overcharge (overvoltage), overdischarge(undervoltage) and excessive charge and discharge currents (overcurrent,short circuit), information of particular importance in Li-ion batterysystems. In certain embodiments, a battery monitor can assume some ofthe functions of a protection circuit by protecting the battery fromharmful overcharging and overcurrent conditions.

FIG. 2 is an exemplary block diagram illustrating the generalfunctionality provided by a battery system and method for monitoring andbalancing battery packs. A battery pack 102 can include one or moreenergy delivery devices 104 (e.g., lithium cells), electricallyconnected in series and/or in parallel.

A voltage monitoring module 106 can receive voltage informationassociated with each of the energy delivery devices 104, condition andisolate the voltage information and provide the voltage information viaan output port to a system controller 108. The system controller 108 canevaluate each of the energy delivery devices 104 during charging anddischarging to determine if any individual energy delivery device 104 isin a potentially damaging state. For example, during charging, anindividual cell may reach or exceed a safe voltage level even though theoverall pack voltage is still below a safe level. Similarly, duringdischarge, the voltage of an individual cell may drop below a minimumsafety threshold even though the voltage of the overall battery pack isstill above its minimum safety threshold. In such events, the systemcontroller 108 can discontinue charging or discharging the battery pack102 (or the individual cell) when the system controller 108 detects anindividual cell voltage at an undesired value.

A balancing module 110 can evaluate the relative voltage levels ofadjacent cell pairs and redistribute charge between the adjacent cellsto mitigate differences in the cell voltages of the pair. As will bedescribed in more detail below, the balancing module 110 can includefunctionality for preventing excessive cell discharge of one cell in theevent the other of the cell pair is removed or otherwise disconnected.

A temperature monitoring module 112 can receive informationcorresponding to the temperature of the battery pack 102. Thetemperature information can be in the form of an electrical signalproduced by a thermocouple located within the battery pack, although thetemperature information can take other forms known in the art. Thetemperature monitoring module 112 can provide the temperatureinformation to the system controller 108. The temperature monitoringmodule may also receive temperature information from other battery packs(not shown) and provide the temperature information from multiplebattery packs to the system controller 108. This description is providedfor the purpose of illustration only and in not intended to be limitingof the invention.

3. Battery Charging Monitoring

FIG. 3 illustrates the use of a reference electrode in monitoring abattery (single cell or battery pack) during a charging event. Duringcharging the battery, the positive electrode can be delithiated and thenegative anode can be lithiated to provide a desired state of charge tothe cell. As noted above, the desired state of charge may be a fullycharged state (100%) or some intermediate value (40-60% for HEVs),depending on the battery application.

In a particular embodiment, the cell can include a reference electrodeand a pair of working electrodes and can be configured to measure thepotential difference of the negative electrode and the referenceelectrode, as shown in step 310.

The information can be provided to a controller and the information canbe compared with a predetermined voltage. For example, as shown in step320, the voltage sensor can provide the potential difference across thenegative electrode and the reference electrode to the controller andthis value can be compared against a critical threshold voltage forwhich plating of lithium at the anode can occur. In certain embodiments,the critical threshold is set at a value greater than zero vs. Li|Li⁺,as the resistance between the working and reference electrodes canaffect the measurement of the potential difference between the workingand reference electrodes. This higher threshold can help to ensure thatthe potential at the negative electrode surface in contact with theelectrolyte does not become low enough to permit lithium metal plating.By way of example, a relatively conservative criterion may be toterminate charge before the voltage between the reference electrode andthe negative current collector reaches 0.01 volts minus the equilibriumpotential of the reference electrode versus Li. In other instances thelower limit of the voltage between reference electrode and negativecurrent collector at which lithium plating does not occur can bedetermined experimentally.

If the potential difference is above the critical threshold, no actionis taken and the system can continue to monitor the potential at thenegative electrode (arrow 325).

If the potential difference is at or below the critical threshold, thecontroller can check the state of charge of the cell (step 330) todetermine whether the target state of charge has been attained (step340).

If the estimated SOC equals the target SOC, charging is deemed to becomplete and charging can be terminated (step 350).

However, if the estimated SOC is less than the target SOC, charging cancontinue.

In certain embodiments, the charge rate can be lowered (step 360) toavoid a further reduction in the negative electrode potential so as toavoid lithium plating. Higher charge rates can result in greater ohmicdrop in the electrolyte and can lower the potential below that at whichlithium plating occurs, as described more fully in U.S. Pat. No.7,262,979, the contents of which are incorporated by reference herein intheir entirety.

Accordingly, monitoring the potential difference between the negativeelectrode and the reference electrode while charging can allow thecharging process to be terminated before lithium plating occurs.

4. Optimizing Battery Charging Time

FIG. 4 illustrates another embodiment of the invention in which areference electrode is utilized to minimize charge time. The chargecurrent that is applied to charge the battery at any particular SOC canbe optimized in real time and the charging profile can essentially beadjusted instantaneously. The reference electrode can allowinstantaneous feedback of the SOC of each individual cell to the batterymonitoring system and allows for real-time adjustment of the chargingprotocol. In contrast, conventional charging protocols cannot beadjusted in real time during a charging event by measuring thepotentials at the electrodes.

As shown in step 410, the electrochemical cell can be configured tomeasure the potential difference between the negative electrode and thereference electrode. The measured potential difference can be providedto a controller, and the information can be combined with otherinformation to estimate the state of charge of the cell, as shown instep 420. The estimated state of charge can be compared to a pre-storedstate of charge profile in step 430. A state of charge (SOC) profile canbe stored in the controller (or any other suitable medium) and caninclude data indicating the maximum permissible charge rate at a givenstate of charge. The maximum permissible charge rate can be determinedfrom a number of factors, such as the potential reached at the negativeelectrode, cell temperature (in order to avoid overheating), and othersafety factors. The SOC profile may be generated using empirical data,such as the measured or known variation in the negative electrodepotential with SOC, or it may be calculated using a model of processessuch as lithium diffusion in the negative electrode. If the estimatedSOC is equal to or about the target SOC (step 440), then the batterycharging can be terminated (step 450). If the estimated SOC is less thanthe target SOC, then the charge rate may be adjusted upwards ordownwards to maintain a charge rate that provides one or more of optimalsafety operation and optimal charge rate to minimize charge time (step460). The charge current may be intermittently raised or lowered(pulsed) to determine the cell potentials that will result at adifferent current.

Accordingly, certain embodiments of the invention provide real-timeadjustment of the charging step to minimize battery charging time. Forinstance, the charging time for a PHEV pack could be minimized by havingcontinuous feedback to the charging source. Minimal charge time may beachieved because the maximum permitted charge rate (the maximum may bedetermined based on a variety of factors such as safety, polarization,etc.) is used for all SOC. By monitoring each cell in a battery pack,the system may be able to compensate for manufacturing variability ofcells as well as changes over life. In other embodiments, pulse chargeand intermittent current reversals may be implemented to avoid buildingup concentration gradients within the electrolyte or either electrodethat can cause the potential to approach undesirable magnitudes. Thus,the reference electrode allows one to operate closer to the edge ofmaximum allowable charge rate.

EXAMPLES

The invention is illustrated in the following examples which arepresented by way of example only and are not intended to be limiting ofthe invention.

Example 1 Li Ion Prismatic Cell Using Li Reference Electrode

A prismatic cell was made by stacking a negative electrode 522,separator 526, and positive electrode 524, as shown in FIG. 5. A copperwire 536, insulated except for the tip, was placed adjacent to (but notin electrical contact with) the negative electrode 522, such that it wascovered by the separator 526 but not between the active area of theanode 530 and cathode 532. Lithium metal 534 was then rolled onto theexposed end of the copper wire. The cell was filled with electrolyte(LiPF₆ in a mixture of carbonate solvents) and hermetically sealed. Thecell went through some conditioning cycles at room temperature, then wasplaced in a Tenney temperature chamber and brought to −20° C. The cellwas cycled on an Arbin battery cycler. FIG. 6 shows a voltage profile ofsuch a cell during charge from 50% state of charge at different chargingrates (0.3 C, 0.5 C, 0.7 C, 1 C, and 1.5 C), followed by discharge backto 50% SOC at the 0.7 C rate. The graph shows the potential of thenegative electrode measured against the reference electrode (lowercurve), and also the cell voltage (positive vs. negative electrode;upper curve). The current during charging and discharging operations isshown in the center. The ability of the reference electrode to controlthe charging of the cell is demonstrated. When the potential of thenegative versus reference electrode drops below a predetermined limit,here, 5 mV, the battery monitoring system terminates charging to preventlithium plating. As seen in FIG. 6, initial charging had a maximumpotential of 3.9 V; however, successive discharge/charge cycles showincreasingly smaller maximum potentials. This is because the referencevs. negative electrode reached 5 mV before that point and charging wasterminated.

FIG. 12 shows the benefit of the reference electrode for monitoringstate of charge. The cell was fully charged, then discharged by 5% stateof charge, with each discharge followed by a 2 hour rest. After the cellwas completely discharged, the procedure was reversed, with the cellbeing charged in 5% state of charge increments, with 2-hour rests inbetween. FIG. 12 shows the cell voltage and reference v. negativevoltage versus time (the cell is at 100% state of charge at the left ofthe figure, at 0% state of charge at time=160000 seconds, and again at100% state of charge at the right of the figure). The potential of thenegative electrode has a clear relationship to state of charge. The useof the reference electrode has two benefits. First, the polarization onthe negative electrode relaxes more quickly than the overall cellvoltage, because of the faster diffusion time constant in the negativeelectrode. Therefore, the potential of the negative electrode morequickly relaxes back to its true open-circuit voltage, and there is lesserror from relaxation of polarization when trying to correlate potentialto state of charge. The second benefit is that there is less hysteresisin the potential of the negative electrode vs. reference electrode thanin the overall cell voltage, which contains contributions to hysteresisfrom both the negative and positive electrode materials in thisparticular case. Therefore, the error from hysteresis is lower whenusing the neg v. ref voltage to determine state of charge.

Example 2 Li Ion Prismatic Cell Using Li₄Ti₅O₁₂ (LTO) ReferenceElectrode

Sample A: A reference electrode was prepared by coating a slurry ofLi₄Ti₅O₁₂ (LTO), binder, and conductive additive onto a region of apiece of copper foil, then drying the slurry, calendaring the coating,and cutting the foil into narrow strips, each coated with LTO at one endof the strip. The uncoated part of the copper strip was insulated withtape to create a barrier to electrolyte contact. A prismatic cell wasassembled as described in Example 1, with the additional step that afourth electrode, comprised of Al foil with a patch of lithium ironphosphate, sized to match the LTO reference electrode, was placed on topof the separator covering the LTO reference electrode. After the cellwas filled with electrolyte and hermetically sealed, current was passedbetween the auxiliary lithium iron phosphate electrode and the LTOreference electrode, to activate the LTO reference electrode to itstwo-phase plateau. To activate the Li₄Ti₅O₁₂ reference electrode, aclosed circuit is formed between the reference and external terminalsand sufficient current is allowed to flow such that the referenceelectrode is lithiated to a capacity, e.g., of between 5 and 150 mAh/gwith respect to the Li₄Ti₅O₁₂.

The cell was tested under the HPPC procedure developed by USABC (T. Q.Duong, J. Power Sources vol. 89#2, 244 (2000)). This procedure measuresthe discharge and charge impedance of the battery at every 10% state ofcharge. The cell voltage and the voltage between the reference electrodeand negative electrode (ref. v. neg.) are shown in FIG. 11. The cellvoltage includes impedance from the separator and positive electrode,whereas the ref. v. neg. voltage includes only the impedance from thenegative electrode. FIG. 11 shows that, in this particular cell, theimpedance of the positive electrode and separator are much larger thanthe impedance of the negative electrode, because the voltage change uponchange in current is much larger in the cell voltage than in the ref v.neg. voltage.

A cylindrical cell 1410 is fabricated with a reference electrode 1415prepared as described in Example 2A by coating LTO 1420 onto metal foil1425, as illustrated in FIG. 14A. The uncoated part of metal foil 1425is insulated with tape 1430 to create a barrier to electrolyte contact.FIG. 14B illustrates reference electrode 1440 deposited directly onto aninterior surface of can 1410 according to some embodiments.

Sample B: A prismatic cell is made by stacking multiple repeat units ofnegative electrode, separator, and positive electrode, as described inExample 1. A reference electrode is prepared as described in Example 2Aby coating LTO onto metal foil, e.g. Ni, Cu, or stainless steel foil.Foil uncoated by LTO is insulated by coating it with a materialimpermeable to electrolyte, e.g. polyurethane. The LTO-coated region ofthe reference electrode is covered with a porous, insulating materialthat wicks electrolyte, such as microporous polyethylene, a mixture ofinsulating ceramic particles with binder such as Al₂O₃ with PVDF, orother material commonly used for battery separators. The referenceelectrode is then placed adjacent to the edge of the bottom anode layer.The cell is filled with electrolyte and hermetically sealed. To activatethe Li₄Ti₅O₁₂ reference electrode, a closed circuit is formed betweenthe reference and positive electrode terminals and sufficient current isallowed to flow such that the reference electrode is lithiated to acapacity of between 5 and 150 mAh/g with respect to the Li₄Ti₅O₁₂.

Example 3 Cylindrical Cell Using Cell can as Reference ElectrodeTerminal

A cylindrical cell is made by winding multiple repeat units of negativeelectrode, separator, and positive electrode, as illustrated in FIG. 7.

An exemplary lithium ion battery includes a battery element having acathode and an anode, which are separated by a microporous separatorwhich are tightly wound together and placed in a battery can. A typicalspiral electrode secondary cell is shown in FIG. 7. The secondary cell715 includes anode sheet 701 that includes anode materials coated ontoboth sides of an anode current collector, a separator 702 and cathodesheet 703 that includes cathode material coated onto both sides ofcathode current collector that have been stacked in this order and woundto make a spiral form 709. The cathode sheets 703 include currentcollector leads 705 and the anode sheets 701 include current collectorleads 707. An electrolytic solution is added to the can.

A cylindrical cell including a reference electrode is fabricated asdescribed with reference to FIGS. 7 and 8. The spirally wound cell 709is inserted into a battery can 830. A battery cell can include upper andlower welded end caps. The cell's primary packaging (can and end caps)can be composed of aluminum alloy. The weld seal is typically obtainedby laser welding, or optionally by other metal joining methods such asultrasonic welding, resistance welding, MIG welding, TIG welding. Theend caps of the doubly (upper and lower ends) welded container may bethicker than the can wall; e.g., the end caps may be up to about 50%thicker than the can wall. An anode lead 705 is connected to thenegative terminal 820. The cylindrical cell includes a positive terminal810 located at one end, a negative terminal 820 at the other end, andthe steel or aluminum cylindrical can 830 at floating potential (i.e.,electrically isolated from both electrodes). The cathode lead 705provides electrical contact with the positive terminal 810 and the anodelead 707 provides electrical contact with the negative terminal 820.

To provide a reference electrode without requiring a third terminal toexit the cell, a reference electrode material 840 is applied to theinterior wall of the can, and the exterior of the can is used as thereference terminal, and is no longer at floating potential. In oneinstance, a slurry of Li₄Ti₅O₁₂ as described in Example 2 is applied toan inside surface of an aluminum can in the shape of a line extendingpart or all of the length or circumference of the can. In anotherinstance, a small “patch” of the slurry is applied on the interiorsurface of the can near the negative terminal. The reference electrodeis electronically isolated from the wound electrode by anelectrolyte-permeable polymer or fiber separator, which may in oneinstance be the electronically insulating outer wrap on the woundelectrode, and in another instance may be a separate film applied overthe reference electrode. The Li₄Ti₅O₁₂ reference electrode is activatedas described in Example 2b. Gaskets 850 electrically isolate terminals810 and 820 from can 830.

Providing a reference electrode without requiring a third terminal toexit the cell, as described above, has several advantages. An additionalhermetic feed-through is not required for the reference electrode. Suchan additional feed-through would add a potential electrolyte leak pathto the system, take up additional space, and increase the weight of thebattery. When a reference electrode material is applied to an interiorwall of an electrically conductive can, the use of the exterior of thecan as the reference terminal can simplify cell design.

Example 4 Cylindrical Cell Using Cell can as ReferenceElectrode/Reference Electrode Terminal

A cylindrical cell was manufactured as described in Example 3. Thepositive terminal, negative terminal, and can are all electricallyisolated from each other via insulating polymer gaskets between theterminals and cans. The can is made from aluminum. The aluminum servesas a pseudoreference electrode, which maintains a constant potential aslong as no net charge is passed to the pseudorefence electrode. The cellvoltage is measured between the negative and positive terminals. Inaddition, the voltage between the can and negative terminal ismonitored. Zero net charge can be maintained by also monitoring thevoltage between the positive terminal and can, or by alternating theconnection of leads (first measuring can v. negative terminal, thennegative terminal v. can). FIG. 10 shows the measured voltages during acharge at a rate of 0.2 times the rated cell capacity. The can, actingas pseudoreference, is able to detect that the increase in cell voltageat the end of charge is caused by an increase in the potential of thepositive electrode, while the potential of the negative electrode isconstant at the end of this low-rate charge.

Example 5 Cell Using Additional Third Terminal in Cell Cap for ReferenceElectrode

A cylindrical cell 910 is fabricated with a reference electrode terminal920 through the cell endcap 930, as illustrated in FIG. 9. This terminal920 is electronically insulated from the endcap 930 and from theterminal passing through the endcap. On the interior of the cell, thereference electrode terminal is electrically connected to a referenceelectrode. In one instance, the reference electrode is Li₄Ti₅O₁₂,fabricated and electronically isolated as described in Examples 2 and 3,and is positioned immediately adjacent to the active area of thejellyroll adjacent to the anode tab.

Example 6 Battery Monitoring System Using Three Electrode Cell

A battery pack contains multiple modules wired in parallel, each moduleconsisting of a string of series-wired cylindrical lithium ion cells,said cells having a doped nanoscale lithium iron phosphate cathode, acarbon anode, and a lithium titanate reference electrode as described inExample 3. Electronic circuits on each module are used to performindividual cell balancing, fault detection, and temperature monitoring.The main monitoring system for the pack performs module fault signalaggregation, dc bus voltage and current monitoring, pack state-of-chargeestimation, module enabling and external fault and status reporting. Thevoltage between the lithium titanate reference electrode and the carbonnegative electrode, V₁, as well as the cell voltage, is monitored ateach cell.

The battery pack is charged using a constant voltage, current-limited DCpower source with an upper current limit of 10 A to each module. Themaximum charge current is monitored using the V₁ output of the cells ineach module.

During charging of the pack, each module in the battery pack isinitially charged with a current of 10 A as long as the modulemonitoring system indicates that the initial SOC of all cells in themodule is below 85%. The initial state-of-charge is determined accordingto a look-up table defining a specific state-of-charge with a specificV₁ value under cell open-circuit conditions. The SOC of cells isdetermined periodically from the voltage V₁ under open circuitconditions. As the SOC of any one cell in a module reaches 85%, thecharge current to the module is decreased to 7.5 A. At 90% SOC thecurrent to the module is reduced to 5 A, and at 95% the current isreduced to 1 A. The module charge current is shut off when V₁ in any oneof its series-connected cells has increased above 1.55 V (for an LTOreference electrode which is 1.56 V positive of lithium). For overchargeprotection, if V₁ in any one of the cells in a module increases above1.55 V for 10 seconds while charging, an error condition is reportedfrom that module to the main monitoring system, which then reports amodule failure and logs the incident for later diagnostics.

Cell balancing is also accomplished using the output V₁. The controlcircuit in each module works to balance the V₁ values of each of thecells with each other. The balancing circuits are designed to dissipateabout 250 mA of excessive charge current around each cell in theinstance of cylindrical cells of 26650 form factor. When the cells haveequal V₁, the balancing current settles to zero.

Impedance growth in each cell is monitored periodically and the datarecorded by the module or main monitoring system. Individual cellimpedance is monitored by charging or discharging each module ofseries-connected cells with a current pulse and determining thepotential drop at the positive and negative electrodes from themeasurement of V₁ for the cell and the cell voltage. The voltage dropdivided by the charge or discharge current provides an impedance valuefor both electrodes in each cell. This data is recorded periodicallyduring the life of the pack.

For pack diagnostics, queries from a diagnostic reader can ask andreceive the following information from the battery pack:

-   -   Number of presently failed modules (if any), based on the        V₁>1.55 V for 10 s criterion    -   Other Alerts or Issues (battery impedance, high temperature,        high charge current, etc.)    -   Temperature history    -   Cell voltage history    -   Hours of operation    -   Estimated impedance (Ω), and growth since date of manufacture        (%)

The following commands can then be sent to the battery pack

-   -   Enable Balancing    -   Disable Balancing    -   Reset module Fault states    -   Reset diagnostic data (history)

Other Embodiments

It is to be understood that while the invention has been described inconjunction with the detailed description thereof, the foregoingdescription is intended to illustrate and not limit the scope of theinvention, which is defined by the scope of the appended claims. Otheraspects, advantages, and modifications are within the scope of thefollowing claims.

1. A battery, comprising: first and second working electrodes separatedby at least one separator, the first working electrode in electricalconnection with a first terminal, and the second working electrode inelectrical connection with a second terminal; one or more referenceelectrodes; and a can housing the working electrodes and the one or morereference electrodes, wherein the can is electrically isolated from thefirst and second terminals and is electrically connected to the one ormore reference electrodes to provide terminals for the one or morereference electrodes and exposed metal surfaces of the can are coatedwith a nonporous electrically insulating coating.
 2. The battery ofclaim 1, wherein the battery is a lithium ion battery and the workingelectrodes comprise electroactive materials capable lithium uptake andrelease.
 3. The battery of claim 1, wherein the battery comprises acylindrical cell of wound construction.
 4. The battery of claim 1,wherein the battery comprises a prismatic cell of wound or stackedconstruction.
 5. The battery of claim 2, wherein the one or morereference electrodes are comprised of an electroactive material capableof multiphase existence to provide a substantially constant voltagebetween about 1 V and about 4 V with respect to Li/Li⁺.
 6. The batteryof claim 2, wherein the one or more reference electrodes are capable ofinterface with a battery management system for the charging of thebattery and for monitoring state of charge.
 7. The battery of claim 2,wherein the one or more reference electrodes have as little as about0.001% and up to as much as about 20% of the coulombic capacity of theworking electrodes.
 8. The battery of claim 2, wherein the one or morereference electrodes are selected from the group consisting of lithiumtitanium oxide, lithium transition-metal phosphate, lithium manganesespinel, with or without substituent elements, and alloys of lithium withmetals such as tin, aluminum, and antimony.
 9. The battery of claim 2,wherein the battery is one of a plurality of batteries comprising abattery pack.
 10. The battery of claim 2, wherein the one or morereference electrodes are positioned at a location in the battery mostsusceptible to lithium plating during charge.
 11. The battery of claim2, wherein the one or more reference electrodes are located between theworking electrodes.
 12. The battery of claim 2, wherein the one or morereference electrodes are substantially adjacent to the edge of thenegative electrode, and prevented from contacting the negative electrodeby a porous, electronically insulating layer.
 13. The battery of claim2, wherein the active material for the one or more reference electrodesis coated onto at least part of a wall of the can.
 14. The lithium-ionbattery of claim 13, wherein the can consists of a metal from the groupcomprising aluminum, copper, stainless steel, and titanium and the canprovides both the reference electrode and the reference electrodeterminal.
 15. The battery of claim 2, wherein the first and secondterminals are located in upper and lower cover plates, respectively. 16.The lithium-ion battery of claim 15, wherein the first and secondterminals are electrically isolated from the can via gaskets.
 17. Thebattery of claim 2, wherein the one or more reference electrodes arewrapped in a porous electronically isolating material that iselectrochemically inert.
 18. The lithium-ion battery of claim 17, wherethe porous electronically insulating material is wetted by the batteryelectrolyte.